1. Field of the Invention
The present invention relates to a grafted polymer electrolyte membrane, a method of producing a grafted polymer electrolyte membrane, and a fuel cell comprising a grafted polymer electrolyte membrane. In particular, the grafted polymer electrolyte membrane of the present invention has excellent stability during use and has good adhesion when used as a solid state polymer electrolyte in a fuel cell or water electrolysis cell.
2. Discussion of the Background
Solid state polymer electrolytes are used in electrodialysis cells, diffusion dialysis cells, as cell diaphragms, and for other applications. Solid state polymer electrolytes are solid state polymer materials which have an ion conductive functional group, such as a sulfonic acid group or carboxylic acid group, which bonds strongly with specific ions and selectively transport cations or anions, depending on the nature of the functional group. A solid state polymer electrolyte may be formed into any shape, including particles, fibers, and flat sheet or hollow fiber membranes.
A solid state polymer electrolyte offers many advantages over conventional electrolytes. For example, proton-exchange membrane fuel cells which have solid polymer electrolyte membranes have higher energy output densities (allowing the fuel cell to be smaller and lighter), few or no problems with electrolyte scatter because the electrolyte is a solid, simplified pressure control because of the high resistance of the solid polymer electrolyte to pressure differentials across the electrolyte, reduced corrosion of fuel cell materialsxe2x80x94hence greater fuel cell durabilityxe2x80x94by the solid polymer electrolyte membrane, and the ability to operate the fuel cell at a lower temperature.
The basic operating principals of proton-exchange membrane fuel cells are explained with reference to FIG. 1, which is an embodiment of a grafted polymer electrolyte membrane according to the present invention. A pair of electrodes 12 and 14 are provided on opposite sides, respectively, of a solid state proton-exchange electrolyte membrane 10. The anode or fuel electrode 12 is supplied with a fuel gas composed of pure hydrogen or reformed hydrogen, and the cathode or air electrode 14 is supplied with an oxidizing gas composed of oxygen or air. The following reactions occur at the electrodes, and the resulting movement of protons (hydrogen ions) in the polymer electrolyte membrane generates an electromotive force.
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: xc2xdO2+2H++2exe2x88x92xe2x86x92H2O
As shown above, water is generated in the cathode reaction. Particularly when the current density of the cell is large, the cathode may become covered with water generated by the cathode reaction (i.e., xe2x80x9cfloodingxe2x80x9d), which interrupts the flow of oxidizing agent (oxygen or air) at the cathode. This flooding phenomenon can therefore degrade fuel cell performance.
Various types of ion exchange membranes, for example polyphenolsulfonic acid membranes, polystyrene sulfonate membranes, and polytrifluorostyrene membranes have been used as proton-conductive solid-state electrolyte membranes. At present, perfluorocarbon membranes are the most commonly used.
Conventional perfluorocarbon membranes have a perfluoroalkylene polymer main chain (or xe2x80x9cbackbonexe2x80x9d), and are not crosslinked. Thus, the side chains of the perfluorocarbon polymer, which contain proton-conductive functional groups, have a relatively high degree of freedom (i.e., are quite mobile). In addition, when such membranes are ionized, the main chain is highly hydrophobic, whereas the proton-conductive side chains are highly hydrophilic. Nafion(trademark) (made by DuPont) is a typical example of such a membrane. Such fluorocarbon membranes have high chemical stability due to their chemical structure (i.e., predominantly Cxe2x80x94F bonds), and have therefore been developed for, and studied under severe conditions.
However, fluorocarbon-type polymer electrolyte membrane are difficult and very expensive to make. Thus, fluorocarbon-type polymer electrolyte membranes have been limited in use to special applications such as space or military proton-exchange membrane fuel cells, with the result that fluorocarbon-type polymer electrolyte membranes have not been widely used for non-military applications.
Various attempts have been made to provide solid state polymer electrolyte membranes which have comparable or improved properties compared to fluorocarbon-type polymer electrolyte membranes, and which are also much less expensive to produce. For example, Nezu et al (U.S. Pat. No. 5,994,426, which corresponds to Japanese Laid-open Patent No. Hei. 09(1997)-102322) proposes a sulfonic acid type polystyrene-graft-ethylene-tetrafluoroethylene copolymer membrane (a sulfonic acid type ETFE-g-PSt membrane) which has a main chain composed of a fluorocarbon-type copolymer and a hydrocarbon-type side chain having a sulfonic group. The ETFE-g-PSt membrane is inexpensive to produce, has sufficient strength when fabricated as a thin film, and the electrical conductivity can easily be controlled by adjusting the type and amount of proton-conductive functional group introduced. The ETFE-g-PSt membrane is a useful material which has both high mechanical strength and high electric conductivity, which results from the combination of a side chain polymer having proton-conductive functional group graft-polymerized into to the highly crystalline main chain polymer.
The results of the above studies show that solid state polymer electrolyte membranes have improved properties over conventional electrolyte membranes.
In addition to having high mechanical strength and electrical conductivity, a solid state polymer electrolyte membrane should have high permeability to water and good adhesion at the interface of the electrode and the solid polymer electrolyte membrane. In particular, the fuel cell performance depends largely on how well the solid state polymer electrolyte membrane adheres to the electrode.
The relationship between the adhesion of the solid polymer electrolyte membrane to the electrode, and fuel cell performance is thought to be the following. In addition to the inherent permeability properties of the solid state polymer electrolyte membrane material to water, the interface or boundary layer between the solid state polymer electrolyte membrane and the electrode also affects the mobility of water in the fuel cell. In general, if the polymer of the solid state polymer electrolyte membranes has a strongly hydrophobic main chain, the solid state polymer electrolyte membrane does not adhere well to the electrode, and therefore does not provide a good connection at the interface between the electrode and the solid state polymer electrolyte membrane. The resulting poor connection at the interface between the electrode and the solid state polymer electrolyte membrane causes increased resistance at this interface. As a consequence, the benefits of using a solid state polymer electrolyte membrane cannot be realized (i.e., the ability to use a thinner, higher surface area membrane with low electrical resistance).
In order to avoid such problems, the solid state polymer electrolyte membrane is generally surface treated prior to incorporation into the fuel cell in order to improve adhesion of the solid state polymer electrolyte membrane to the electrode. For example, the surface of the membrane is sand blasted or chemically etched to form fine irregularities in the surface, and to increase the surface area. However, fluorocarbon-type polymer electrolyte membranes are difficult to chemically modify, and therefore it is difficult to improve the adhesion properties of such materials.
In general, solid state polymer electrolyte membranes, when formed by a membrane-forming device, have a thickness which ranges from 100 to 200 xcexcm. However, conventional methods of making such thin membranes results in thin films of uneven thickness. This uneven thickness can also cause poor contact between the electrode and the surface of the electrolyte membrane, thereby increasing the electrical resistance of the cell where the membrane film is thicker. In addition, mass production lowers the efficiency and increases the cost of solid state polymer electrolyte membranes. Thus, reducing the boundary resistance at the electrode/polymer electrolyte-membrane interface is essential to increasing the performance of proton-exchange membrane fuel cells.
In order to avoid such problems, Japanese Laid-open Patent No. Hei. 4(1992)-220957 describes an improved method for increasing fuel cell performance and adhesion of the electrolyte membrane, in which a solid state polymer electrolyte membrane is plasma treated, to provide a thinner film with even thickness and a rough surface. The plasma treated membrane has a larger surface area for contacting the electrode, resulting in a solid state polymer electrolyte membrane fuel cell which has lower internal resistance.
In addition, Japanese Laid-open Patent No. Hei.5 (1993)-258756 describes a technique for increasing the fuel cell performance, wherein the surface of a roughened solid state polymer electrolyte membrane is sputtered coated with a metal layer which provides more reaction sites.
However, if the above-described hydrocarbon electrolyte membrane or fluorocarbon-type electrolyte membranes are plasma treated in oxidative atmosphere, employing the method described in Japanese Laid-open Patent No. Hei. 4(1992)-220957, the proton-conducting functional groups (i.e., sulfonic acid or carboxylic acid groups) are also oxidized. Since the proton-conducting functional groups play an important role in the diffusion of oxygen and water in the fuel cell, oxidation of these functional groups unavoidably lowers the performance of the fuel cell.
In addition, if the surface of the membrane is roughened by plasma treatment and then sputter coated with metal, as described in Japanese Laid-open Patent No. Hei.5 (1993)-258756, the thermal damage to the surface of the membrane resulting from the plasma treatment is greater than expected. Even though the electric conductivity is improved by the sputter coated metal layer at the interface between the membrane and the electrode, the production cost of such fuel cells is high. Thus, it difficult to obtain good fuel cell performance at low cost.
Thus, there is a need for an electrolyte membrane, and methods for producing them, in which inherently hydrophobic polymers, which are therefore less adhesive than other polymers, are provided with improved adhesion. Fuel cells containing such polymer electrolyte membranes should have lower electrical resistance at the interface between the electrode and the membrane, which thereby reduces flooding, and provides excellent operating stability. Thus, ultimately, the present invention is directed to providing improved fuel cells, so that fuel cells can be used in new applications such as electric vehicles.
It is therefore a primary object of the present invention to provide a solid polymer electrolyte fuel cell which meets such a need.
In order to attain this object, the electrolyte membrane of the present invention comprises:
a precursor membrane which is capable of being graft polymerized, which is pre-treated by plasma treatment in an oxidative atmosphere;
a side chain polymer is then graft-polymerized to the precursor membrane; and
a proton conductive functional group is introduced to the side chain polymer.